Chalcone synthesis is a frequent experiment in sophomore organic lab courses and 4-methylacetophenone and 4-methylbenzaldehyde are often the reactants. Since the methyl group releases electrons to the ring while the carbonyl withdraws electrons from the ring, the chemical shifts of the C2 and C3 are extremely close. SDBS and other databases state that these signals cannot be assigned with certainty. However the H atoms attached to these Cs have very distinct shifts because of the anisotropic effect of the C=O group. Therefore a CH correlation spectrum allows this assignment to be made unequivocally. These 13C assignments may be done also by the conceptually simpler 1H coupled 13C spectrum. Previously we have shown that this latter strategy works with toluene even though the 2D approach fails there.

Teaching interpretation of 1H and 13C NMR spectra independently of each other can fail to reach insights achieved by considering them together

254th National Meeting (2017)

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